Heartworm hotspots: When pet travel is the trouble

In its annual parasite forecast, released in April, the Companion Animal Parasite Council predicted that the incidence of heartworm disease would be higher than average across the country this year. At Fetch dvm360 conference in Baltimore, parasitologist Brian Herrin, DVM, PhD, discussed the impact of animal transport on shifting heartworm prevalence in the United States, noting that while “heartworm infection isn't moving much on its own, it does seem to be moving because of a lot of travel.”

Long-distance shelter animal movement

A significant amount of animal travel involves long-distance transport of shelter animals. “These dogs surely deserve a happy life,” explained Dr. Herrin, an assistant professor at Kansas State University College of Veterinary Medicine, “but we have to consider the diseases that travel with them.”

Those counties that have accepted the highest number of shelter dogs have also reported the largest increase in heartworm-positive dogs, and animal movement undoubtedly helped transplant the disease. Between 2014 and 2017, for example, more than 130 animal welfare organizations imported about 114,000 dogs to Colorado shelters, turning the area into a heartworm disease hotspot.

Most states don't legally require heartworm testing before interstate travel. Also, the disease can become established quickly after arrival, Dr. Herrin warned, because “every state has mosquito vectors that are capable of transmitting heartworm.” The American Heartworm Society (AHS) and Association of Shelter Veterinarians (ASV) recently published procedures for minimizing the risk of heartworm transmission during animal travel. Additionally, the two societies stated: “Along with considering the recommendations in this document, veterinarians should ensure that transportation of animals is carried out in accordance with state and/or federal transportation regulations, as well as professional guidelines.”

AHS and ASV recommendations

The AHS/ASV document states that all dogs should be tested for heartworm disease before travel. If testing isn't possible or test results aren't immediately available, then relocation should be postponed until testing can be completed.

If a dog tests heartworm-positive, treatment should begin right away, before the animal travels, particularly because infection can leave a dog too unstable for travel. However, if treatment is impossible at the time, then at minimum a macrocyclic lactone preventive and doxycycline therapy should be administered. “Infected dogs are a risk to the surrounding area,” Dr. Herrin stated, and administering these drugs helps both the patient and the population. By preventing circulating microfilariae from growing into adult worms, a preventive reduces the patient's worm burden and stops disease transmission to other animals. Doxycycline reduces inflammation from dying worms.

Also, a topical permethrin-based insecticide product is recommended before travel to prevent disease transmission by disrupting the mosquito life cycle. Upon arrival, the AHS-recommended treatment protocol should be instituted. Retesting after arrival is not recommended, because preventive and doxycycline treatments can both cause false-negative antigen test results.

Dogs that test heartworm-negative before travel may still suffer from prepatent infection. For these dogs, a preventive should be administered before travel and retesting should be performed 6 months later. In the meantime, according to Dr. Herrin, preventive should continue for “every pet, every month, all year round.”

Dr. Stilwell received her DVM from Auburn University, followed by a MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida. She provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting.